Pressure relief device

ABSTRACT

A circular seal ( 19 ) for the end face of a substantially cylindrical canister, the seal ( 19 ) having a substantially rectangular cross-section, and a recess ( 21 ) in one side thereof to reduce the area of said cross-section. In use the recessed area ( 21 ) of the seal ( 19 ) constitutes a weakened portion which will blow out elastically to relieve excess pressure.

[0001] This invention relates to a means of relieving excess pressure inan air braking system of a vehicle, and particularly to a means ofrelieving pressure in the desiccant canister of an air dryer.

[0002] Air braking systems of vehicles incorporate an air dryer betweenthe air compressor and the air consumers. This dryer usually comprises avalve block and a replaceable closed canister of desiccant material.Typically the canister consists of a steel pressing in the form of acup-like body and into which the desiccant is loaded, and an end plateconnected to the body by means of a rolled seam. Certain internalpartitions may be provided.

[0003] Typically, the end plate has concentric inlet and outletapertures and is attached to valve block by means of a female screwthread. A peripheral elastomeric ring provides the necessary sealbetween the end plate and the valve block.

[0004] The construction of such a container is also well known inrelation to replaceable oil filters of vehicles.

[0005] Safety devices are provided in the system to ensure that theoutput pressure of the compressor is restricted to a safe level. Usuallythe compressor is unloaded by a control mechanism before the safetylimit is reached, and a separate pressure relief valve provides a secondlevel of safety protection at a predetermined limit pressure.

[0006] There is a very small possibility that the compressor unloaderand the pressure relief valve may simultaneously fail to operate,perhaps due to poor maintenance of the system. In these circumstances arise in pressure may cause a failure of one of the components is the airdryer canister, not least because air pressure acts over a relativelylarge cross-sectional area, and the forces developed can be ratherlarge. For example a nominal pressure of 12 bar acting on a canistercross-sectional area of 10000 mm² results in a net force in excess of 1tonne.

[0007] What is required is a means of relieving excess pressure at thedesiccant container at a low cost, in a controllable manner andpreferably in a way which is re-settable after the excess pressure hasbeen released.

[0008] According to the invention, there is provided a circular seal forthe end face of a substantially cylindrical canister, the seal having asubstantially rectangular cross-section, and a recess in one sidethereof to reduce the area of said cross-section.

[0009] In use the recessed area of the seal constitutes a weakenedportion which will blow out elastically to relieve excess pressure. Theseal is preferably adapted to re-seat to permit continued operation ofthe vehicle, but alternatively the seal can remain in the blowncondition as a visual indication of a fault. In the latter case the sealis adapted to be re-seated by slightly unscrewing the canister wherebythe inherent resilience of the seal will cause it to adopt theundistorted condition. The canister may then be re-tightened toreinstate the integrity of the air system.

[0010] It is envisaged that deformation of the seal during a blow outwill reduce slightly the elasticity thereof so that repeated blow-outswill occur at successively lower pressures if the fault is not repaired,and eventually a minimum operating pressure will not be maintainedwithin the system.

[0011] Conventionally failure to reach a minimum pressure is indicatedto the driver by a cab buzzer or the like.

[0012] By permitting the seal to re-seat, or to be reseatable, thevehicle is not immediately disabled, but can continue its depot where apermanent repair may be effected.

[0013] Preferably the recess extends radially of the seal, and in thepreferred embodiment is provided on the radially inner side of the seal.The recess may reduce the radial wall thickness to 50-80% over apredetermined arc. In one preferred embodiment the recess is in the formof an arc. In an alternative embodiment the recess has a substantiallycircumferentially extending wall at the base thereof. A recess may beprovided both on the inner and on the outer side of the seal. Severalrecesses may be provided, and may be symmetrical and/orcircumferentially aligned. The recess may be provided on an axiallyfacing side of the seal, either wholly or partially; two or morerecesses may be provided in order to ensure that the seal is not handed.

[0014] In preferred embodiment means are provided to indicate that theseal has blown. Such means may comprise coloured fluid arranged to markvehicle components in the vicinity of the canister. The fluid ispreferably contained in a burstable sac around the seal or in microsacscontained in the seal in the vicinity of the weakened portion(s).

[0015] Other features of the invention will be apparent from thefollowing description of a preferred embodiment shown by way of exampleonly in the accompanying drawings in which:

[0016]FIG. 1 is a schematic elevation of an air dryer canister and valveblock, partially in section.

[0017]FIG. 2 is an enlarged scrap section showing a rolled edgeconnection.

[0018]FIG. 3 shows in plan a seal according to the invention;

[0019]FIG. 4 shows an enlarged partial plan of the seal of FIG. 3;

[0020]FIG. 5 shows an enlarged partial plan of an alternative seal; and

[0021]FIG. 6 is an elevation of the seal of FIG. 5 from the inside.

[0022]FIG. 1 illustrates a valve block (10) having an inlet passageway(11) and outlet passageways (12). The inlet and outlet passageways mayalternatively be reversed. A male threaded connection partially definesthe inlet passageway (11).

[0023] A replaceable canister comprises a hollow cup-like shell (13)closed by an end plate (14). The shell and plate are rolled at theperimeter, as shown in FIG. 2, to give a leak tight connection. The endplate has a female threaded inlet opening (15), and a plurality ofoutlet openings (16) for communication with the outlet passageways (12).

[0024] Within the canister a perforated cylindrical screen (17) retainsan annular charge of desiccant material (18). A circular seal (19) ofrectangular cross-section seals the end plate (14) to the valve block(10).

[0025] In use the canister is supplied with air under pressure at 7-12bar, the air being dried as it passes through the desiccant material(18) to the outlet passageway (12). There is no significant pressuredrop through the material (18), and accordingly the threaded connectionis sufficient to separate inlet and outlet flow. The external seal (19)is however essential to maintain the connection between canister and thevalve block leak free.

[0026] A portion of the circumference of the seal (19) is relieved, asillustrated by the arcuate recess (21) of FIGS. 3 and 4. This weakenedportion is adapted to relieve excess pressure within the canister bydistortion, in use the weakened portion allowing excess pressure to ventto the exterior. By careful design, the inherent resilience of the sealis arranged to permit re-seating of the seal by manually unscrewing thecanister to a small degree and re-tightening, thus permitting continuedsafe operation of the braking system. In the event that the seal doesnot re-seat, the usual low pressure warning signal will alert thevehicle driver that insufficient pressure is available for brakeoperation.

[0027] The seal may be provided with a special peripheral form to ensurelocation on the valve body in a predetermined orientation. Thisarrangement ensures that high pressure venting can be arranged in a safedirection, and/or that release of an indicator, dye or the like can bein a preferred direction.

[0028]FIG. 5 illustrates an alternative embodiment comprising a circularrectangular section seal 22 having a half depth recess 23 on the upperside as viewed. The recess has a circular inner face 24 terminating ateither end in a substantially right angled corner with a convex blendingsurface 25. The seal is for example 113 mm in diameter and the recessextends for less than 10% of the circumference. Several such recessesmay be provided around the periphery of the seal at equal spacing, andrecesses may be provided in both the upper and lower surfaces as viewed.

[0029]FIG. 5 also illustrates somewhat schematically a microsac 26incorporated in the seal 22 in the vicinity of the recess, and adaptedto burst on significant deformation of the seal as may occur duringunseating thereof. The microsac 26 contains a coloured fluid which mayspray or drip over adjacent components in order to indicate that theseal has blown.

1. A circular elastomeric seal for the end face of a substantiallycylindrical canister, the seal having a substantially rectangularcross-section, and a recess in one side thereof to reduce the area ofsaid cross-section, thereby to provide a weakened portion susceptible ofradial deformation in the event of a predetermined radial pressuredifferential.
 2. A seal according to claim 1 wherein the recess is in acircumferentially extending face of the seal.
 3. A seal according toclaim 2 wherein the recess is in an inner face of the seal.
 4. A sealaccording to any preceding claim wherein the recess is in an axiallyfacing side of said seal.
 5. A seal according to any preceding claimwherein said recess is arcuate.
 6. A seal according to any of claims 1-4wherein said recess has a circumferentially extending wall.
 7. A sealaccording to any of claims 1-6 wherein said recess comprises less than25% of the cross-sectional area of said seal.
 8. A seal according to anypreceding claim wherein said recess has a depth less than 50% of thethickness of said seal in one of the radial and axial directions.
 9. Aseal according to any preceding claim and further including a sac ofcoloured fluid, said sac being in the vicinity of said recess andburstable on deformation of said weakened portion.
 10. A seal accordingto claim 9 wherein said sac is within said rectangular cross section.